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Friday, September 28, 2012

10 effects you should have heard of

The Photoelectric Effect

Light falling on a metal plate can lead to emission of electrons, called the "photoelectric effect". Experiments show for this to happen the frequency of the light needs to be above a threshold depending on the material. This was explained in 1905 by Albert Einstein who suggested that the light should be thought of as quanta whose energy is proportional to the frequency of the light, the constant of proportionality being Planck's constant. Einstein received the Nobel Prize in 1921 "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."

This effect was first predicted by Hendrik Casimir who explained that, as a consequence of quantum field theory, boundary conditions that may for example be set by conducting (uncharged!) plates, can result in measurable forces. This Casimir force is very weak and can be measured only at very small distances.

The Doppler effect, named after Christian Doppler, is the change in frequency of a wave when the source moves relative to the observer. The most common example is that of an approaching ambulance, where the pitch of the signal is higher when it moves towards you than when it moves away from you. This does not only happen for sound waves, but also for light and leads to red- or blueshifts respectively.

Electrons in a conducting plate that is brought into a magnetic field are subject to the Lorentz force. If the plate is oriented perpendicular to the magnetic field, a voltage can be measured between opposing ends of the plate which can be used to determine the strength of the magnetic field. First proposed by Edwin Hall, this voltage is called the Hall voltage, and the effect is called the Hall effect. If the plate is very thin, the temperature low, and the magnetic field very strong, a quantization of the conductivity can be measured, which is also known as the quantum Hall effect.

The Meissner-Ochsenfeld effect, discovered by Walther Meissner and his postdoc Robert Ochsenfeld in 1933, is the expulsion of a magnetic field from a superconductor. Most spectacularly, this can be used to let magnets levitate above superconductors since their field lines can not enter the superconductor. I assure you this has absolutely nothing to do with Yogic flying.

A charged particle in an electromagnetic field acquires a phase shift from the potential of the background field. This phase shift is observable in interference patterns and has been experimentally confirmed. The relevant point is that it's the potential that causes the phase, not the field. Before the Aharonov–Bohm effect one could question the physical reality of the potential.

The Hawking Effect

Based on a semi-classical treatment of quantum fields in a black hole geometry, Stephen Hawking showed in 1975 that black holes emit thermal radiation with a temperature inverse to the black hole's mass. This emission process of the black hole is called the Hawking Effect. This result has lead to a great progress in understanding the physics of black holes, and is still subject of research, see recent post at Cosmic Variance.

In the presence of a magnetic field, energy levels of electrons in atomic orbits that are usually degenerated (i.e. equal) can obtain different values, depending on their quantum number. As a consequence, spectral lines corresponding to transitions between these energy levels can split into several lines in the presence of a static magnetic field. This effect is named after the Dutch physicist Pieter Zeeman, who was awarded the 1902 physics Nobel prize for its discovery. The Zeeman effect is an important tool to measure magnetic fields in astronomy. For some historical reasons, the plain vanilla pattern of line splitting is called the Anomalous Zeeman effect.

The Mikheyev-Smirnov-Wolfenstein effect, commonly called MSW effect, is an in-medium modification of neutrino oscillation that can for example take place in the sun or the earth. It it a resonance effect that depends on the density of the medium and can significantly effect the conversion of one flavor into another. The effect is named after Stanislav Mikheyev, Alexei Smirnov and Lincoln Wolfenstein.

The Sunyaev-Zel'dovich effect, first described by Rashid Sunyaev and Yakov Zel'dovich, is the result of high energy electrons distorting the cosmic microwave background radiation through inverse Compton scattering, in which some of the energy of the electrons is transferred to the low energy CMB photons. Observed distortions of the cosmic microwave background spectrum are used to detect the density perturbations of the universe. Dense clusters of galaxies have been observed with use of this effect.

Named after the Austrian theoretical physicist Wolfgang Pauli, the Pauli Effect is well known to every student of physics. It describes a spontaneous failure of technical equipment in the presence of theoretical physicists, who should therefore never be allowed on the vacuum pumps, lasers or oscilloscopes.

22 comments:

What is definition of the "effect"? In particular, does some conceptual difference exist between "effect" and "law"? For example, is the rotation of the Earth around Sun a manifestation of "Copernicus effect" or "Kepler's law"?

Thanks as I remember this from the first time you presented it. Of all the effects there are two I find particularly interesting, with the first being the Hawking effect, as from what I understand it being the only one on your list that hasn’t been actually experimentally confirmed and to no wonder as how does one practically take the temperature of a black hole; or am I wrong on that. The other is the Aharonov–Bohm effect for as you say it demonstrates the physical reality of a potential, which I find interesting as a potential being thought of as what J.S. Bell would have called a beable is not something found to be generally accepted in physics as they thought more of just as convenient conceptual tool rather than something to be taken as real.

As with many words, I don't think there's a strict definition, there's more a vague context in which the word might be used. I'd say a law is usually some relation, in the stricter sense an equation, while an effect is a statement about a specific physical consequences of such equations. Like the "frame-dragging effect" is a consequence of Einstein's field equations. Like the Hawking effect is a consequence of qft in curved space, and so on. Best,

Well, there's many more effects than those, but I think there's a merit in not drowning readers in information. And, yes, the frame-dragging effect is a good one, of course. Maybe it would deserve to be listed instead of the MSW effect, which is really a quite specialized topic. Best,

Yes, of course, there are many more effects and I was not suggesting you to be exhaustive. It's just that I am particularly fascinated with gravity/GR, and I thought one particularly nice representative at least should be mentioned.

I realize it may be a little stale for you and some other readers who have been around for a longer time. But it seems to me that our readership has changed quite a bit and so I thought it would be worthwhile. You might have noticed that the Aharonov–Bohm effect is a newcomer to the list and replaces the butterfly effect, which didn't really fit very well. It was suggested in the comments to the earlier version. I recall being amazed by the Aharonov–Bohm effect for so nicely giving a concret physical meaning to the rather abstract idea of integrating around a loop. Best,

as a retired electronics engineer who for the last (active)20yrs worked whith physicists I can assure you that the only effect that is 100% certain, observable, and needs no mathematical proof is what you call the Bonus Effect.

It can however be deducted from the Murphy's laws, but this wisdom is largely forgotten these days.

Right. There's been some back and forth over whether the Hawking effect has been seen in analogue gravity models. I haven't really followed that, but I think people are still debating whether it is what it's supposed to be. Either way, while the Hawking effect is generally about particle production in qft and so has these "analogues" , it is more specifically about black hole evaporation and there are of course no observations for that. Best,

I hope you didn’t think I implied this post to be redundant, that is even in respect to your long term readership as that certainly wasn’t my intent. Also I did take note that the Aharonov–Bohm effect to be added and thought it to be an excellent choice for reasons I’ve already given. That is I’ve always found the whole concept of potential fascinating, as to wonder what has the machine we call the universe able to be recognized to be found distinguishable from its reason.

Hello Bee, why did You not use the Term Hallwachs-Effect Your first exammple? An Effect is a observation/experiment with an not obvious outcome. For this reason the term "Hawking-Effect" is not appropriate. RegardsGeorg

Consider the Yarkovsky effect and its second order Yarkovsky-O'Keefe-Radzievskii-Paddack effect. They render obsessive orbital plotting of near-Earth objects less then rigorous over time. Distorted non-spherical and small objects do quite a dance in sunlight over time. 99942 Apophis the Uncreator, 270 meters in diameter, need only boogie over a skosh on its Friday 13 April 2029 visit to splatter the Earth on Hot Fudge Sunday 13 April 2036 (NASA-estimated 510 megatons).

That being substantially after the fall of Western civilization, get ready to kickstart a new religion. Angry God ho!

I'm not so sure I agree with your attempt at defining the term "effect".

From the samples chosen by Bee and by the discussion it is clear, I think, that "effect" is used in physics in a very vague and colloquial way. Yes, sometimes effects are observations, but sometimes they can be theoretical results (waiting to be observed).

Rather than imposing some strict definition, which risks de-effecting phenomena commonly called effects, it would be more useful to make a broad survey of the actual use of the term, and then try to extract the meaning from those findnigs.

So: an effect is what is called an effect. No need to apply nomenclatorial rigour in situations where it would obscure understanding rather than improve it.

With regards to the Pauli effect, do German speakers still refer to "die tucke des objekts"? I ran into the phrase while reading an account of why Fritz Haber, of nitrogen fixation fame, didn't want to give a demo of his lab apparatus back in 1909. The book I read translated the phrase as "the spitefulness of things", apparently a familiar phrase in laboratory circles.